In order to reduce the landing distance of a jet engine powered aircraft, as well as to increase the margin of safety when the aircraft is landing on a wet or icy runway, thrust reversers are utilized on the jet engines in order to provide a braking thrust for the aircraft. Such thrust reversers function to reverse the direction of the jet thrust, from a normally rearward direction used for propelling the craft, to a forward direction for slowing or braking the craft.
Typically the thrust reversers are formed by thrust reverser "doors" are hingedly mounted on a fixed structure called a jet pipe, and usually form the final nozzle of the gas turbine engine. The doors are capable of pivoting between two positions about an axis which is transverse and substantially diametrical with respect to the jet of the engine.
The first position finds the doors in a stowed position, out of the direct path of the exhaust blast of the engine. In this position, the doors form the exhaust nozzle of the gas turbine engine so that the thrust of the engine is directly rearward, thereby producing the forward thrust of the aircraft. In the second position, the doors are pivoted about the pivot axis to a transverse, blast deflecting or deployed position, to intercept and redirect the jet blast and produce the braking thrust for the aircraft when needed. In order to allow such pivotal movement in prior art thrust reversers, the trailing edge of the reverser doors must be fully scarfed to enable the pivotal deployment and the butting of the trailing edges of the two doors against one another in the deployed position. This scarfing produces what is known as a "fishmouth" shape at the exhaust outlet because of its appearance when viewed from the side. Such a "fish-mouth" opening is technically considered as non-planar.
Experience has shown that this shape rather significantly degrades the engine performance in a forward thrust mode of operation, and this degradation is caused by two combined factors. Firstly, there is a rather significant loss of nozzle efficiency because of efflux spillage sideways through the cut away portions created by the scarfs. Secondly, there is a significant thickness of the trailing edge of the reverser doors, leading to an excessive base area, in turn leading to a non-negligible base drag.
In addition, the thrust reversers according to the prior art patents significantly change the inner flow lines of the jet stream through the engine, since the throat of the nozzle is (and must be) located upstream of the trailing edge of the reverser nozzle. Also, from the throat to the trailing edge of the reverser, the inner skin of the door has a cylindrical or divergent extension. Irrespective of the fishmouth exit, or the cut away portions created by the scarfs (which significantly degrade the engine nozzle performance), the structure extending aft of the throat of the nozzle increases the nozzle discharge coefficient at all engine pressure ratios. This in turn modifies the engine operating characteristics. Therefor, with this type of thrust reverser nozzle, it is necessary to modify the basic aircraft performance manual.
The prior art demonstrates some variable area exhaust nozzles for thrust reversers, and such devices are shown in U.S. Pat. No. 4,966,327 and French Patent 2,614,939. Theoretically, a variable area exhaust provides certain benefits for the jet engines, in enabling them to adapt to different conditions of the flight. For example, since different characteristics of the engines are desirable for take off, climb and cruise modes of operation, and varying the exhaust area enables the engine to better achieve the optimum characteristics.
According to the prior patents mentioned above which integrate a variable exhaust area nozzle with the thrust reverser, the thrust reverser doors do not (and cannot) in any of the controllable positions of the reverser doors define the throat of the exhaust nozzle to be at the trailing edge of the nozzle/reverser doors. In these prior patents, the inner flow line of the thrust reverser doors is convergent toward the throat, with a cylindrical extension aft of the throat. This means that at any position of the thrust reverser nozzle, the throat of the nozzle always remains significantly upstream of the reverser nozzle trailing edge. This is an inherent characteristic in the design of the prior art as it is described in French patent 2,382,594.
In the case of the prior art thrust reversers using the scarfed or fishmouth nozzles, another drawback arises in that the performance characteristics of the engine are somewhat degraded such that varying the exhaust area merely helps to recapture the efficiency lost to the scarfing. This degradation of performance is caused by a not-insignificant lateral efflux of exhaust gas in the area of the fishmouth cutouts.
In U.S. Pat. Nos. 4,194,692 and 4,093,122, the thrust reverser doors are incapable of defining, in any of their controllable positions, the throat area of the nozzle. By the term "throat" as it is commonly used in this field is meant the point or location of the minimum cross-sectional area of the engine's ejection nozzle. In both of these prior patents, the throat area of the nozzle is not adjusted by the thrust reverser doors, but rather by a separate variable convergent nozzle.
In addition, the fishmouth cutouts of prior art thrust reversers necessarily affect the configuration of the exhaust nozzle of the engine upon which they are installed, even when in the stowed position. For example, the exhaust nozzle cannot be a strictly convergent nozzle, which is desirable for better cruise performance. In the prior art, the throat of the nozzle is necessarily at the boundary of the convergent portion and of the cylindrical or divergent fishmouth extension, i.e. upstream of the fishmouth exit. This changes the engine operating characteristics in comparison to a basic, non-reversing engine nozzle.
Accordingly, a primary object of the present invention is to provide an improved thrust reverser for jet aircraft engines.
Another object of the invention is to provide a thrust reverser which does not interfere with normal engine operating characteristics.
Still another object of the invention is to provide a thrust reverser which does not affect the jet flow lines of the engine.
Yet a further object of the invention is to provide an improved thrust reverser in which when in the stowed position, the inner skin of the thrust reverser doors and the half shells form the flow boundary of the jet of the engine.